U.S. patent number 3,715,495 [Application Number 05/119,136] was granted by the patent office on 1973-02-06 for signal seeking type auto-tuning television receiver.
This patent grant is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Sukeichi Miki, Tsuneo Takezaki.
United States Patent |
3,715,495 |
Takezaki , et al. |
February 6, 1973 |
SIGNAL SEEKING TYPE AUTO-TUNING TELEVISION RECEIVER
Abstract
A signal seeking type auto-tuning television receiver including
a tuning means which has a reversely biased voltage variable
capacitance diode, a main detecting means which comprises a limiter
with a filtering circuit and a frequency discriminator for
detecting sound IF carrier signals and developing a main control
signal, a subsidiary detecting means which comprises a tuned
amplifier, a transistor detector and a rectifier for detecting
picture IF carrier signals and developing a subsidiary control
signal, and a scanning means coupled to the tuning means to supply
a sweep voltage to the voltage variable capacitance diode and
controlled by the main and subsidiary control signals so as to stop
when a desired television signal is received and so as not to stop
when undesired noises are detected.
Inventors: |
Takezaki; Tsuneo (Neyagawa,
JA), Miki; Sukeichi (Hirakata, JA) |
Assignee: |
Matsushita Electric Industrial Co.,
Ltd. (Osaka, JA)
|
Family
ID: |
22382737 |
Appl.
No.: |
05/119,136 |
Filed: |
February 26, 1971 |
Current U.S.
Class: |
348/732; 334/15;
455/162.1; 331/36C; 334/20 |
Current CPC
Class: |
H03J
7/24 (20130101) |
Current International
Class: |
H03J
7/24 (20060101); H03J 7/18 (20060101); H04n
005/62 () |
Field of
Search: |
;178/5.8A,5.8R,5.8F
;325/471 ;334/15,20 ;331/36C,177U |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Griffin; Robert L.
Assistant Examiner: Stout; Donald E.
Claims
What is claimed is:
1. In a signal seeking type auto-tuning television receiver having
a tuning means including a reversely biased voltage variable
capacitance diode in a signal selection circuit and which converts
a selected television signal to picture and sound intermediate
frequency signals, an intermediate frequency amplifier which is
coupled to said tuning means and is provided with a predetermined
band pass characteristic within which said picture and sound
carrier signals are correctly converted to intermediate frequency
signals, a stable D.C. power supply, and a sound intermediate
frequency amplifier, the combination comprising:
a scanning means coupled to said tuning means, said scanning means
having means for supplying a sweep voltage to said voltage variable
capacitance diode for scanning through the frequency spectrum of a
television band;
a driving means coupled to said scanning means for driving said
scanning means;
a control circuit means coupled to said driving means for
controlling said driving means;
a main signal detecting circuit means connected with said IF
amplifier, said means including a limiter having a filtering
circuit for rejecting undesired signal components and further
including a frequency discriminator having a resonant circuit tuned
to a first predetermined frequency for detecting sound IF carrier
signals which are in said predetermined frequency and for
developing a main control signal at the output of said frequency
discriminator;
a subsidiary signal detecting means connected in parallel with said
main signal detecting circuit means, said means including a tuned
amplifier tuned to a second predetermined frequency, a detector
coupled to said tuned amplifier and having a resonant circuit tuned
to a synchronizing signal frequency for detecting the picture IF
carrier signal at said second predetermined frequency, and a
rectifier connected with said detector through said resonant
circuit and for developing a subsidiary control signal;
a gate circuit means connected between said main signal detecting
means and said control circuit means, said means being controlled
by said subsidiary control signal for feeding said main control
signal to said control circuit means; and
a mechanical switching means coupled to said control circuit means,
said control circuit means starting and stopping a signal seeking
action through an action of said driving means according to the
operation of said mechanical switching means and developing said
main and subsidiary control signals.
2. The combination as claimed in claim 1 wherein said gate circuit
means comprises an active element having an output terminal
connected between said main signal detecting means and said control
circuit means and an input terminal connected with an output
terminal of said subsidiary signal detecting means, and the output
conductance of said active element is controlled by said subsidiary
control signal applied to said input terminal for feeding said main
control signal to said control circuit means only when said
subsidiary control signal is present.
3. The combination as claimed in claim 1 wherein said control
circuit means comprises a bistable multivibrator and a switching
circuit means coupled thereto, and said bistable multivibrator
actuates said switching circuit means for starting or stopping said
driving means.
4. The combination as claimed in claim 1 wherein said scanning
means comprises a variable resistor and a switching means, said
variable resistor comprising a substrate, at least one
semi-circular resistance element on said substrate, a first
resistor connected between one terminal of the resistance and the
stable D.C. power supply, and a second resistor connected between
the other terminal of the resistance and ground, a shaft, and a
slider fixed on said shaft and contacting said resistance element
and having a terminal connected to said voltage variable
capacitance diode in said tuning means, whereby said voltage
capacitance diode is reversely biased by a voltage related to the
rotational position of said shaft, and said switching means
consisting of another slider fixed on said shaft and rotated
together with said slider of said variable resistor, a further
substrate, and at least one metallic electrode on said further
substrate and having a similar shape to said resistance element,
said switching means being connected between said D.C. power supply
and said tuning means for supplying D.C. power to said tuning means
only when said slider of said variable resistor contacts said
semi-circular resistance element and for activating said tuning
means only during the aforesaid time interval.
5. The combination as claimed in claim 4 wherein said variable
resistor in said scanning means has two semi-circular resistance
elements arranged opposite each other on the substrate for scanning
a low and high TV channel in the VHF band, respectively, said
resistance elements each having two terminals and being connected
to the D.C. power supply and ground through resistors,
respectively, and said switching means having two semi-circular
metallic electrodes.
6. The combination as claimed in claim 1 wherein said frequency
discriminator in said main signal detecting circuit has two output
terminals, one of said two output terminals being for said main
control signal and said frequency discriminator developing a signal
for automatic frequency control at the other of said output
terminals, the other output terminal being coupled to said tuning
means.
7. The combination as claimed in claim 1 further comprising a
switching element which is coupled to said control circuit means,
said control circuit means having a bistable multivibrator, said
switching element being controlled by an output voltage of said
bistable multivibrator of said control circuit means and being
coupled to the sound intermediate frequency amplifier for
controlling the action of said sound intermediate frequency
amplifier so that sound amplification is stopped during a signal
seeking period.
Description
This invention relates to a signal seeking type autotuning
television receiver system using a reversely biased voltage
variable capacitance diode and an electro-mechanical driving means
for changing said bias voltage.
Several signal-seeking tuning systems for TV receivers have been
proposed heretofore, which systems use continuously variable tuning
elements such as variable air-capacitors or variable capacitance
diodes in the tuning means, electromechanical driving means or
electronic sweeping means by which the reactances of the tuning
elements are continuously varied for scanning the frequency
spectrum, and a signal detecting means to detect the presence of a
signal being received and to stop the driving means or the sweeping
means when the continuously variable tuning elements are tuned to
the desirable received TV signals.
Tuning systems of this type have the capability of easy and all
channel selection in both the VHF and UHF bands.
But conventional signal seeking type auto-tuning television
receivers are apt to cease their signal seeking action and cause
the production of an undesirable picture and sound upon a reception
of a noise signal, such as an image signal, a city noise, a pulse
noise produced by a motor, and so on. They also have another
undesired receiving condition, that in which they receive only one
carrier signal of two carriers, i.e. the picture or the sound
carrier, instead of the signals of two carriers, i.e. the picture
and the sound carrier, and stopping the signal-seeking action when
this one carrier signal is received.
Accordingly, it is an object of this invention to provide an
improved signal seeking system for TV receivers.
It is another object of this invention to provide an improved
signal seeking system which distinguishes between picture and sound
carriers by an innovative circuit arrangement, and converts picture
and sound carriers to respective picture and sound IF signals
correctly positioned within a predetermined IF bandpass
characteristic so as to properly tune in a desired TV signal.
It is a further noted object of this invention to provide an
improved signal seeking system which distinguishes between desired
TV signals and undesirable noise or image signals and receives only
desired TV signals.
In accordance with this invention, an auto-tuning television
receiver is provided which includes a main signal detecting circuit
means and a subsidiary detecting circuit means. The main signal
detecting circuit means is coupled to an intermediate frequency
amplifier and detects an IF carrier signal which is at a
predetermined frequency at which a sound IF carrier signal is to be
present, so that it normally detects a sound IF carrier signal. The
subsidiary detecting circuit means is also coupled to the
intermediate frequency amplifier and detects the presence of a
picture IF carrier signal.
In one embodiment of this invention, said main signal detecting
circuit means comprises a tuned amplifier which is tuned at the
sound IF carrier frequency and which also acts as a limiter and a
frequency discriminator. The subsidiary signal detecting circuit
means comprises a tuned amplifier which is sharply tuned at the
picture IF carrier frequency, an AM detector and a synchronizing
signal separator.
Furthermore, the auto-tuning television receiver system according
to this invention includes a gate circuit means which is actuated
by a subsidiary control signal to control a control circuit means.
The subsidiary control signal is developed in the subsidiary
detecting circuit means.
Further objects, novel features, and attending advantages of this
invention will be pointed out in the following description and
claims, and illustrated in the accompanying drawings which
disclose, by way of example, the principle of the invention and the
best mode which has been contemplated of applying that
principle.
In the drawings:
FIG. 1 is a schematic circuit diagram, partly in block form, of an
auto-tuning television receiver system in accordance with this
invention, wherein some portions similar to the conventional
television receiver system are omitted, for example, a C.R.T., a
depletion circuit, etc., since these omitted portions do not play
any part in the signal seeking function of the system according to
the invention;
FIG. 2A is a graph showing the IF amplifier bandpass characteristic
of a typical television receiver;
FIG. 2B is a graph showing the output frequency characteristic of a
main signal detecting circuit means;
FIG. 2C is a graph showing the bandpass characteristic of the tuned
amplifier in subsidiary detecting circuit means;
FIG. 3 is a diagram of an internal arrangement of a variable
resistor with a switching means;
FIG. 4 is a circuit diagram showing the connection of a scanning
circuit means with said variable resistor;
FIGS. 5A-5D are graphs explaining the operation of the scanning
circuit means including the variable resistor and the switching
means in one embodiment of the invention, FIG. 5A showing the
relationship between the state of the switching means and the
rotational position of a contact of the variable resistor, FIG. 5B
and FIG. 5C showing the relationships between the output voltages
and the rotational positions for the VHF band and the UHF band,
respectively, of the contacts of the variable resistor, and FIG. 5D
also showing the relationship between the output voltage and the
rotational position of the contact of the variable resistor in the
further improved scanning circuit means; and
FIGS. 6A and 6B are graphs showing the relationships between the
tuned frequency of the receiver and the rotational position of the
contact of the variable resistor, FIG. 6A being for a conventional
receiver and FIG. 6B being for the improved receiver of the present
invention.
Now referring to FIG. 1, which is a schematic circuit diagram for
an all channel television receiver, an antenna 1 and a tuner 2 are
provided for VHF band tuning, and an antenna 1' and a tuner 2' are
provided for UHF band tuning.
Television signals in a VHF band intercepted by the antenna 1 are
supplied to the tuner 2. The tuner 2 comprises a resonant circuit
13, a local oscillator 15 and a mixer circuit 17. The resonant
frequency of the resonant circuit 13 is governed by a voltage
variable capacitance diode 14 and the oscillatory frequency of the
local oscillator 15 is governed by a variable capacitance diode 16.
The mixer circuit 17, which is coupled both to the resonant circuit
13 and the local oscillator 15, mixes the received TV signal and
the output signal of the local oscillator so as to provide an
intermediate frequency component.
The scanning circuit means 12 comprises a DC power supply 56, a
variable resistor 18 and a switching means 19, and it reversely
biases the voltage variable capacitance diodes 14 and 16.
The functions of the parts of the receiver for UHF band tuning are
designated by primed reference numbers, that is, the antenna 1',
resonant circuit 13', mixer 17', local oscillator 15', and voltage
capacitance diodes 14' and 16', and are similar to the
corresponding parts for VHF band tuning, respectively; that is, the
antenna 1, the resonant circuit 13, the mixer 17, the local
oscillator 15 and the voltage capacitance diodes 14 and 16. The
tuner 1 and the tuner 1' are selectively actuated by a band
selection switch 57.
The variable resistor 18 and the switching means 19 are ganged so
as to be moved together and are driven by a driving means 11. The
driving means 11 comprises a motor 20, a gear 21, and a driving
amplifier 22.
In the scanning circuit means 12, the variable resistor 18 is
coupled to the D.C. power supply 56 through biasing resistors 23
and 24, and is grounded through other biasing resistors 25 and 26,
so that a terminal (c) of the variable resistor 18 is supplied with
the voltage for biasing the variable capacitance diodes 14 and
16.
A resistor 27 is connected between the terminal (c) and the ground,
and the function thereof will be described in detail hereinafter,
in relation to the description of the structure and operation of
the variable resistor 18 and the switching means 19.
The intermediate frequency signals are amplified by an intermediate
frequency amplifier 3 and detected by a video detector 4. The
output signal of the detector 4 includes a video signal, and it is
amplified by a video amplifier 5 and supplied to a C.R.T. (which is
not shown in FIG. 1).
An intercarrier sound signal which is produced by the interaction
of the picture and the sound intermediate frequency signals is also
amplified by an amplifier 28 within the video amplifier 5 and then
supplied to a sound intermediate frequency amplifier 6.
Further, the amplified sound IF output of the amplifier 6 is
detected and fed to a speaker through an audio amplifier. In FIG. 1
the following means and their connections in a conventional TV
receiver are not shown; i.e., a cathode ray tube, an audio
amplifier, a speaker, a deflection control circuit, a synchronizing
signal separator, a power supply circuit, chromatic control
circuits in the case of a color TV, etc., because they are not
directly related to the subject matter of the invention.
The composite video IF carrier signal from the intermediate
frequency amplifier 3 is also supplied to the main signal detecting
circuit means 7. The main signal detecting circuit means 7 includes
two filters 29 and 30, a tuned amplifier 31 and a discriminator 32.
The two filters 29 and 30 will be described in detail hereinafter.
The tuned amplifier 31 has a resonant circuit which is tuned, for
example, to a frequency of 41.25 MHz, which is the predetermined
proper intermediate frequency of the sound carrier signal for the
present TV receivers in the U.S.A.
The tuned amplifier 31 also functions as a limiter when the input
signal has a large amplitude.
The discriminator 32 has a resonant circuit tuned to 41.25 MHz and
two output terminals (d) and (e) balanced to the ground. The
terminal (e) is connected to a terminal of the voltage variable
capacitance diode 16 included in the local oscillator 15 through an
electric switching circuit means 33 for providing an automatic
frequency control action. The other terminal (d) of the
discriminator 32 is supplied with a main control signal from the
main signal detecting circuit 7 and it is connected to a gate
circuit means 9.
A portion of the output composite video IF carrier signal from the
intermediate frequency amplifier 3 is also applied to a subsidiary
signal detecting circuit means 8.
The subsidiary signal detecting circuit means 8 includes a tuned
amplifier 36, a detector 37, and a rectifier circuit means 38
connected in this order. The tuned amplifier 36 has a resonant
circuit which is sharply tuned to a frequency of 45.75 MHz, which
is the predetermined proper intermediate frequency of the picture
carrier signal for the present TV receivers in the U.S.A. The
detector 37 has a resonant circuit tuned to 15.750 KHz,
corresponding to the frequency of the horizontal synchronizing
pulse.
The horizontal synchronizing signal pulse is rectified and smoothed
by the rectifier circuit means 38 which includes a diode 39 and a
smoothing condensor 40.
Therefore, when the picture IF carrier signal is provided at the
input of the subsidiary signal detecting means 8, a negative
voltage (i.e. subsidiary control signal) appears at terminal (f)
through a resistor 41.
The gate circuit means 9 comprises a switching transistor 34, a
variable resistor 35 for determining a switching level, and a
resistor 42.
The collector of the transistor 34 is connected to both the output
terminal (d) of the discriminator 32 and an input terminal of a
control circuit means 10. The emitter of the transistor 34 is
grounded and the base is connected to both the output terminal of
the subsidiary signal detecting means 8 and to the D.C. power
supply through a variable resistor 35. The switching transistor 34
is normally in the "on" state, because the base is sufficiently
biased through the variable resistor 35. But, when sufficient
negative output voltage is supplied to the base of the transistor
34 from the output of the rectifier circuit means 38, the switching
transistor 34 is switched to the "off" state, and the main control
signal from the main signal detecting means 7 is fed to the control
circuit means 10.
The control circuit means 10 comprises a first bistable
multivibrator 43, a second bistable multivibrator 44, a switching
circuit means 45 and mechanical switches 48 and 49 which are
actuated by a viewer.
The switching circuit means 45 is connected in parallel with the
second bistable multivibrator 44. Upon being actuated by the output
voltage of the first multivibrator 43, the switching circuit means
45 controls an output signal flow of the second multivibrator
44.
The state of the first bistable multivibrator 43 is changed by the
mechanical switch 48 or 49 and the output voltage of the main
signal detecting circuit means 7. When the mechanical switch 48 or
49 is pushed, the first bistable multivibrator 43 causes the
switching circuit means 45 to open so as to complete the signal
flow path from the second bistable multivibrator 44, and at the
same time it actuates the electronic switching circuit means 33 so
as to stop the automatic frequency control action. When the output
voltage of the main signal detecting circuit 7 appears, the first
bistable multivibrator 43 is changed to the other stable state, and
it reverses the state of the switching circuit means 45 and 33 to
cause the receiver to receive the desired television signal.
The stable state of the second bistable multivibrator 44 is
determined by the mechanical switching means 48 and 49, which
determines the direction of rotation of the motor 20 or the
direction of tuning. For example, if the switch 48 is pushed, the
output voltage of the second bistable multivibrator 44 is applied
to the driving amplifier 22 through the switching circuit means 45.
Then, the output signal of the driving amplifier 22 actuates the
motor 20 and thus actuates the scanning circuit means 12 in the
normal direction, i.e. from the low to the high tuning frequencies
within a television band.
The driving means 11 comprises the driving amplifier 22, the motor
20 and the gear 21. The motor 20 is connected to the driving
amplifier 22 and is supplied with a voltage therefrom. The polarity
of the voltage is determined by the second bistable multivibrator
44. The gear 21 is ganged mechanically with the motor 20 so as to
transmit the rotation of the motor 20 to the shaft of the variable
resistor 18 in the scanning circuit means 12 while reducing the
speed of the rotation.
In the auto-tuning television receiver system in accordance with
the invention, the driving means is stopped by the two control
signals from the main and subsidiary signal detecting circuit means
8 and 9, i.e. the main and subsidiary control signals. The main
control signal is supplied from the discriminator 32, wherein a
part of the signal having a frequency of 41.25 MHz from the IF
amplifier 3 is amplified and limited by the tuned amplifier 31
before it is applied to the discriminator 32. Therefore, it is very
seldom that the main control signal is erroneously provided just
because of the presence of any undesirable noise, such as a city
noise, a motor noise, or the noise produced at the time of starting
and finishing scanning through the TV band. But, the main control
signal is developed by the presence of either the picture or the
sound IF carrier signal at the frequency of 41.25 MHz during the
scanning of the frequency spectrum.
However, in the subsidiary detecting circuit means 8, it is only
when the electric waves containing the synchronizing signal come in
that a subsidiary control signal is developed by the transistor
detector 37 having the resonant circuit tuned to the horizontal
synchronizing signal frequency 15.75 KHz. Accordingly, it is only
when the presence of the picture IF carrier signal very nearly at
the predetermined frequency (45.75 MHz in the case of present TV
receivers in the U.S.A.) is detected that the subsidiary control
signal is developed.
The gate circuit 34 is opened by the subsidiary control signal, and
only then is the main control signal applied to the first bistable
multivibrator 43.
Accordingly, the trigger signal to the first bistable multivibrator
43 is developed only when the picture IF carrier signal is very
nearly at the predetermined frequency of 45.75 MHz and when the
sound IF carrier signal is very nearly at the predetermined
frequency of 41.25 MHz.
Therefore, the signal seeking action of the tuning means 2 is
stopped only when the picture and sound carrier signals are
correctly positioned within the predetermined bandpass
characteristic of the IF amplifier 3, and is not stopped when only
one of the picture or sound intermediate frequency carrier signals
is within the bandpass characteristic of the IF amplifier. The
action is also not stopped when the tuning means receives an image
of a TV signal, because the frequency relationship of the sound and
picture carrier IF signals is reversed from the correct
relationship.
In FIG. 1, the main signal detecting means 7 includes the filter 30
coupled to the IF amplifier 3 and the tuned amplifier 31, and the
filter 30 includes a capacitor 51. One terminal of the capacitor 51
is connected to both the input terminal of the tuned amplifier 31
and to the IF amplifier 3, and the other terminal is grounded
through an inductance coil 53 and a capacitor 52 connected in
parallel. The resonant frequency of the filter 30, which is
determiend by the inductance of the coil 53 and the total
capacitance of the capacitor 51 and capacitor 52 is set so that it
is about 1 MHz away from the center frequency of the discriminator
32. The anti-resonant frequency which is determiend by the
inductance of the coil 53 and the capacitance of the capacitor 52
is set so that it is between the above resonant frequency and the
center frequency of the discriminator, for developing a sharp main
control signal.
With these connections of the filter 30, the frequency
characteristic of the main signal detecting circuit means 7 is as
illustrated in the left-hand portion of FIG. 2B. Accordingly, the
main control signal becomes very sharp when developed during
scanning of the frequencies and therefore the tuning means is
stopped very nearly at the correct position when a desired TV
signal is being received.
Furthermore, use of this filter 30 provides the advantage of
protection for the low sensitivity of AFC, effected by trapping the
sound IF carrier component in the IF amplifier 3.
The filter 29 is for trapping the picture IF carrier component in
order to obtain AFC action by means of the output voltage of the
discriminator 32 responsive only to the sound IF carrier
signal.
The whole frequency characteristic of the main signal detecting
circuit means 7 containing the filters 29 and 30 is illustrated in
FIG. 2B.
FIG. 2A illustrates the IF bandpass characteristics of a typical TV
receiver used presently in the U.S.A., and FIG. 2C illustrates the
frequency response of the tuned amplifier 36 for comparison with
FIG. 2B.
Referring to FIG. 3, the arrangement of the variable resistor 18
and the switching means 19 in the scanning means 12 is illustrated
schematically for explaining the function thereof in detail
together with FIGS. 4 and 5.
Two resistance elements 71 and 72, which are semicircular types,
are positioned on a substrate 70 and have terminals 73 and 74, and
75 and 76, respectively.
A slider 77 in contact with the resistance elements is rotated by a
shaft 69. The switch means 19 is arranged adjacent to and coaxially
with the variable resistor 18, and has the same axis as that of the
variable resistor 18. A slider 87 is rotated while remaining in
contact with two contacts 85 and 86 which are shaded in FIG. 3.
FIG. 4 is a circuit diagram of an embodiment of the scanning means
using the variable resistor as shown in FIG. 3, wherein the
components corresponding to those in FIG. 3 are designated by the
same reference numerals.
In FIG. 4, a voltage having a saw-tooth shape appears at the
terminal 77 depending on the amount of rotation of the axis, as
shown in FIG. 5B. The resistance element 71 is for scanning the
frequency band in a low frequency channel. The resistors 78 and 79
limit the higher ends of the bias voltage for the voltage
capacitance diodes, and the resistors 80 and 81 limit the lower
ends thereof.
The switches 83 and 84 are the band selecting switches. When the
switches are at a VHF band position, the voltage appearing at the
terminal 77 which is to be applied to the tuning means changes as
shown in FIG. 5B according to the movement of the shaft 69, and the
voltage at a UHF band position is as shown in FIG. 5C. Since the
switch means 19, arranged as shown in FIG. 3, is the power switch
for the tuning means, D.C. power is supplied to the tuning means
only when the slider contact 87 contacts the contacts 85 and 86 to
supply power as at (a) and (b) in FIG. 5A.
Accordingly, the tuning means is not supplied with DC power during
the return time from the end of scanning at the low channel to the
start of scanning at the high channel in the VHF band or from the
end of scanning at the high channel to the start of scanning at the
low channel, and the same in the UHF band.
Therefore, according to this invention, erroneous tuning will not
occur. However, even when using the switching means 19, there is a
problem of inaccurate tuning.
Referring again to FIG. 4, there is provided a resistor 82
connected between the slider terminal 77 of the variable resistor
18 and ground, and it is used for compensating the nonlinearity of
the relation between the tuning frequency and the bias voltage of
the variable-capacitance diode.
If the compensating resistor 82 is not provided, the bias voltage
for the variable capacitance diodes 14 and 16 is applied, as shown
in FIG. 5B, during tuning in the VHF band. Consequently, the tuned
frequency is scanned, as shown in FIG. 6A, because the variable
capacitance diode usually has a non-linear relationship between the
capacitance and the bias voltage. However, there is a problem in
that the control signals have pulses of different shapes between
the lower and the higher portions within the same band (i.e. the
pulse-widths of the signals are different), and consequently the
stopping position of the IF carrier signals within the IF bandpass
characteristic differs, and so accuracy of tuning is not
assured.
The resistor 82 is provided for overcoming the above defects. When
the resistor 82 is provided, the bias voltage applied to the
variable capacitance diode is changed from that as shown in FIG. 5B
to that as shown in FIG. 5D. Accordingly, the scanned frequency is
changed from that as shown in FIG. 6A to that as shown in FIG. 6B.
The resistance value of the resistor 82 depends on the particular
variable capacitance diode which is used.
Thus, by means of a very simple circuit connection, i.e. the
inserting of the resistor 82, the relation between the tuned
frequency and the rotational position of the variable resistor can
be compensated so as to be linear, and, accordingly, much more
accurate tuning is assured.
A switching element 50 is coupled to the sound intermediate
frequency amplifier 6 to control its action. When supplied with the
output voltage of the bistable multivibrator 43, the element 50
stops the operation of the sound intermediate amplifier 6 during
the signal scanning period. Accordingly, a viewer is not disturbed
by noisy sounds during the signal scanning period.
* * * * *